“I have no control over that ad,” Colbert said. “I am not calling anybody a serial killer. I can’t tell Americans for a Better Tomorrow, Tomorrow what to do.”

The ad is meant to be a spoof on the anti-Romney ads being run by a super PAC supporting Newt Gingrich.

Colbert engaged in similar shenanigans during the 2008 presidential race. This time around, he decided to "explore" his presidential prospects after a poll that showed him leading Jon Huntsman in South Carolina.

It's too late to get his name on the ballot in South Carolina, and the state doesn't allow for write-in candidates. But Colbert said he wasn't worried.

"They say I can't get on the ballot in South Carolina?" he told Stephanopoulos. "They said you can't go to the moon. They said you can't put cheese inside a pizza crust, but NASA did it."